During the past few years it was shown that not only carbohydrates, but also free amino acids participate in the metabolic response to myocardial oxygen deprivation. The physiological importance of the breakdown and synthesis of certain amino acids during myocardial ischemia and hypoxia is however not known. A greatly improved perfusion system for the isolated working rat heart has been developed and will be employed to corelate metabolic activities and the physiologic performance of the heart. The system allows to study the removal of substrates from recirculating perfusion medium under stable conditions while cardiac output, left ventricular pressure development and oxygen consumption are similar to those found in the intact animal. It could already be demonstrated that steady state rates of metabolite removal are twice as high as the highest rates reported in the literature. Certain amino acids are also removed by the oxygenated heart, and their metabolic fate will be investigated with the help of enzymatic analysis and column chromatography of medium and tissue. The effect of myocardial ischemia will be investigated in the same perfusion system, however with a lowered aortic impedance. Certain amino acids or their precursors will be supplied in order to test the functional and metabolic capacity of the ischemic heart muscle.